Circuit breaker having improved flashover characteristics



Sept 10, 1968 F L. CAMERON CIRCUIT BREAKER HAVING IMPROVED FLASHOVER CHARACTERISTICS Filed Aug. 24, 1967 VVVVVVVVV/VVWMV//f////// 3 u M United States Patent O v 3,401,245 CIRCUIT BREAKER HAVING IMPROVED FLASHOVER CHARACTERISTICS Frank L. Cameron, Irwin, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug. 24, 1967, Ser. No. 663,029 11 Claims. (Cl. 337-199) ABSTRACT OF THE DISCLOSURE A high voltage circuit interrupter with a fusible means inside a tubular casing and connected at one end with an elongated conducting member movable through a central passageway of arc-extinguishing material. A tubular terminal means extends axially over the end of the casing a predetermined distance along the outside of the casing. A tubular member is connected between the external terminal means and the fusible means and extends axially inside the casing a minimum distance beyond the external terminal means to prevent a ashover externally of the casing. The internal conducting member terminates axially short of substantially the entire fusible means to minimize the overall size of the circuit interrupter.

Background of invention In the application of circuit interrupters, such as power fuses, at higher voltages, such as above 15 kv., a problem arises in preventing llashovers due to the concentration of potential stresses which may result during an interrupting operation. This problem arises in certain types of power fuses intended for application at such higher voltages in which a fusible means is disposed inside an insulating casing or tube and electrically connected to a relatively small conducting rod which is movable through an arcextinguishing means, such as a body of gas evolving ma'- terial, upon the melting of the fusible means to lengthen and extinguish .the arc which results during an interrupting operation. It has been found in certain known power fuse structures that when an arc is initiated and lengthened by the movement of a conducting rod during an interrupting operation, the concentration of potential stress adjacent to certain conducting parts which may be disposed externally of the casing adjacent to the fusible means, such as a conducting end terminal or ferrule, may be sufficient to breakdown the air outside the insulating casing and initiate a iia'shover which may extend the full length of the insulating casing. It is to be noted that during an interrupting operation, the dielectric strength of the medium inside the insulating casing of such a power fuse adjacent to the arc-extinguishing means provided is normally much higher than the dielectric strength of the air or space outside of the insulating casing. In order to prevent such external fia'shovers along the insulating casing of a high voltage power fuse, various constructions have been employed in the past such as disclosed in U.S. Patent No. 2,567,236 which issued Sept. 11, 1951, to H. L. Rawlins et al., and in U.S. Patent No. 2,567,768 which issued Sept. 1l, 1951, to H. H. Fahnoe, which are both assigned to the same assignee as the present application. When a shielding means of the type disclosed in the above patents is employed, the overall length of the fuse structure may have to be increased because of the presence of the shielding means, in order to provide a predetermined insulating gap between the separated conducting parts of the fuse structure between which a potential difference exists following an interrupting operation. This is because a shielding means of the type disclosed in the above patents axially overlaps the associated fusible means prior to an interrupt- ICC ing operation. It is therefore desirable to provide an improved high voltage circuit interrupter or power fuse construction which prevents external ilashovers of the type described while simultaneously minimizing the required di,- mensions or size of the overall fuse structure for a particular voltage rating.

It is an object of this invention to provide a new and improved high voltage circuit interrupter construction.

Another object of the present invention is to provide an improved means for preventing iiashovers in a circuit interrupter due to relatively high potential stresses.

A more specific object of this invention is to provide a high voltage power fuse construction including means for preventing external liashovers due to high potential stresses which permit a more compact overall construction of the high voltage fuse.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description, taken in conjunction with the accompanying drawings, in which: l

FIGURE l is a side elevational view of a? high voltage fuse structure which embodies the principles of the present invention and which is shown in the closed operating condition;

FIG. 2 is an enlarged longitudinal sectional view of a fuse unit which forms part of the fuse structure shown in FIG. l with portions of the end fittings of the fuse unit omitted; and

FIG. 3 is an enlarged view, in perspective, of a tubular conducting member which forms part of the fuse unit shown in FIG. 2.

Referring now the drawings, and FIG. l in particular, the structure shown comprises a' power fuse structure of the high voltage, dropout type, the general arrangement of which is set forth more fully in copending application Ser. No. 663,020, filed Aug. 24, 1967, by R. E. Frink et al. which is assigned to the same assignee as the present application. As shown in FIG. 1, the fuse structure 10 includes a base (not shown) formed of sheet metal and a pair of outwardly extending insulator supports 372 and 382. The upper insulator support 372 fixedly supports in position a latching assembly 360, which includes a break contact 362 as described in greater detail in the copending application just mentioned. The lower insulator support 382 supports a hinge assembly 350 which pivotally supports a fuse unit and which includes a hinge contact 352, as described in greater detail in the copending application previously mentioned. As shown in FIG. 1, the fuse unit 100 serves to electrically bridge the break contact 362 and the hinge contact 352 so that current will normally pass therebetween by way of terminal pads (not shown) to which an external electrical circuit may be connected.

The fuse unit 100 includes a generally tubular fuse holder 32 which is formed from a suitable weatherproof, electrically insulating material, such as a, filament wound glass epoxy material or the like, and a pair of upper and lower end fittings or terminals 34 and 36, respectively, which are disposed -at the opposite ends of the holder 32 and which are formed from an electrically conducting material. The upper and lower end fittings 34 and 36, respectively, are securely fastened to the opposite ends of the associated holder or tube 32 by suitable means such as cement and a plurality of pins which pass transversely through both the end fittings and the associated holder 32. As shown in FIGS. 1 and 2, the fuse unit 100 also includes a hookeye 384 which is pivotally mounted on a laterally projecting portion 34A of the upper end fitting 34 and which may be utilized for effecting opening and closing movements of the fuse unit 100 by means of a 3 conventional hook-stick. The lower end fitting 36 includes a hinge lifting eye 374 which may be formed integrally with the lower end fitting 36 and which may be employed in conjunction with a conventional hook-stick to effect physical removal of the fuse unit 100 from the hinge assembly 350 for replacement of the fuse unit 100. The lower end tting or terminal 36 also includes an inwardly projecting flange portion 36B against which the lower end of the holder 32 bears, as viewed in FIG. 2.

The fuse unit 100 further includes a renewable or refillable unit 20 which is mounted within the holder structure that includes the outer tube 32 and the upper and lower end fittings or terminals 34 an-d 36, respectively. The renewable unit 100 includes its own supporting tube 110 which is formed from a suitable electrically insulating material having sufficient strength to withstand the internal gas pressure which result during an interrupting operation of the fuse unit 100 such as a filament wound glass epoxy material. A body of gas evolving material, such as boric acid, which may include a plurality of generally annular blocks 122, 124, 126 and 128, is disposed inside the tube 110 and spaced from the ends thereof. Each of the blocks 122, 124, 126 and 128 includes a relatively larger central opening and a relatively smaller opening at one side thereof, both of which extend axially therethrough the individual blocks. When the blocks 122, 124, 126 and 128 are axialy stacked in end to end relation as shown in FIG. 2 with the respective larger and smaller openings thereof substantially aligned, a main bore is formed through the body of gas evolving material which includes said blocks, as indicated at 130, and a relatively smaller auxiliary bore is formed through the body of gas evolving material, as indicated at 192.

In order to prevent the travel of ionized gases between the main bore 130 and the auxiliary bore 192 during an interrupting operation of the fuse unit 100, as described in greater detail in copending application Ser. No. 663,018, tiled Aug. 24, 1967, by C. W. Upton, Jr. and I. A. Sensue which is assigned to the same assignee `as the present application, the meeting surfaces of the blocks 122, 124, 126 and 128 are structurally joined to one another around the relatively smaller openings which form the auxiliary bore 192 by a sealing and bonding material having a relatively high dielectric strength, such as an epoxy resin. More specifically, as explained in the copending application just mentioned, the meeting surfaces of the blocks 122, 124, 126 and 128 each includes a recessor groove which extends substantially around the relatively smaller opening in each of said blocks and forms with the adjacent block a combined passageway -which is substantially filled with the sealing and bonding material, as indicated at 132 in FIG. 2. It is to be noted that the manner in which the blocks 122, 124, 126 and 128 are bonded to one another around the auxiliary bore 192 substantially prevents the entrance of the sealing and bonding material into either the auxiliary bore 192 or into the main bore 130.

In order to limit the gas pressure which result during an interrupting operation of the fuse unit 100 inside the tube 110 to a value within the rupture strength of the tube 110, as disclosed in greater detail in copending application Ser. No. 663,126, filed Aug. 24, 1967, by C. C. Patterson which is assigned to the same assignee as the present application, each of the blocks 126 and 128 includes a generally C-chaped recess as indicated at 129 which extends axially fromone end of each of said blocks to a point axially spaced Vfrom the other end of the respective blocks, with each of the recesses 129 terminating peripherally short of the portion of said blocks which include the relatively smaller openings which form part of the auxiliary bore 192. Each of the blocks 12-6 and 128 includes therefore around a major portion of its inner periphery a frangible inner wall, as indicated at 126A and 128A, respectively, which is arranged to disintegrate when the fuse junit 100 is called upon to interrupt relatively 4 e large currents and the gas pressure within the main bore 130v exceeds a predetermined value. During such an interrupting operation, the size of the main bore through the blocks 126 and 128 is effectively increased by the disintegration of the inner walls 126A and 128A of the blocks 126 and 128, respectively, to thereby increase the size of the gas passageway and decrease the gas pressure `which would otherwise result. v v

In order to retain the blocks 122, 124, 126 and 128 in assembled relationship with the associated tube 110, as shown in FIG. 2, the outer surfaces of said blocks may be coated with a suitable cement, such as an epoxy bonding material, prior to assembly of the blocks inside the tube 110. In addition, a generally annular retaining member or plug 189 may be disposed at the `upper end of the blocks 122, 124, 126 and 128 with the major portion of the retaining member 189 extending axially inside the tube 110, as shown in FIG. 2. The retaining member 189 may be formed or molded from a suitable electrically insulating material having suiiicient strength to assist in retaining the blocks 122, 124, 126 and 128 in assembled relationship with the tube during an interrupting operation of the fuse unit 100, such as a glass polyester material. A washer 183 formed from similar material may be disposed between the retaining member 189 and the block 122 and may be employed during the assembly and bonding of the blocks 122, 124, 126 and 128 together prior to assembly of said blocks inside the tube 110. It is to be noted that the retaining member 189, as well as the washer 183, includes a relatively larger central opening which forms an extension of the main bore 130 and a relatively smaller opening which forms an extension of the auxiliary bore 192. In order to assist in retaining the member 189 in lassembled relation with the associated tube 110 during an interrupting operation of the fuse unit 100, the outer surface of the retaining member 189 and the inner surface of the :tube 110 at the upper end of the tube 110 include adjacent helical grooves which together form a passageway in which a heli-cal wire 181 is disposed to firmly secure the retaining member 189 in assembled relation with the tube 110. The retaining member 189 may be assembled with the upper end of the tube 110 by first assembling the helical wire 181 in the groove around the outer surface of the retaining member 189 and then screwing the retaining member 189 into the upper end of the tube 110 to the final position shown in FIG. 2.

In order to substantially prevent the escape of ionized gases from the upper end of the renewable unit 20 around the elongated conducting member 83 which extends through the main bore 130, a generally tubular member 185 is disposed in concentric relation with the retaining member 189, as best shown in FIG. 2, and-is preferably formed of an electrically insulating material having a relatively low coefficient of friction, such as polytetrauoroethylene which is sold under the trademark Teflon The yupper end of the tubular member 185 includes an opening of reduced cross-section through which the conducting member 83 passes and which forms a substantially gas tight seal with the conducting member 83 `during an interrupting operation of the fuse unit 100, as will be explained hereinafter. The tubular member 185 also acts as a bearing to guide the axial movement of the conducting member 83. In order to prevent the tubular member 185 from being blown out the upper end of the tube 110 during an interrupting operation of the fuse unit 100, the retaining member 189 includes an inner shoulder portion against which the upper end of the tubular member 185 bears as shown in FIG. 2. The escape of ionized gases from the upper end of the renewable unit 20 from the auxiliary bore 192 may be adequately prevented by reducing the size of the relatively smaller opening through the retaining member 189 in which the auxiliary conductor 182 is disposed.

In lorder to assist in retaining the blocks 122, 124, 126 and 128 in assembled relationship with the tube 110 during an interrupting operation of the fuse unit 100, a generally tubular retaining member 142 is disposed inside the tube 110 at the lower end of the blocks 122 through 128, as shown in FIG. 2, and is formed or molded from an electrically insulating material having sufficient strength to assist in retaining the blocks 128 inside the tube 110 during such an .interrupting operation, such as a glass polyester material. The -outer surface of the retaining member 142 is preferably coated with a suitable cement, such as an epoxy bonding material, prior to the assembly of the retaining member 142 inside the tube 110. The retaining member 142 includes a relatively larger opening which extends axially therethrough, as indicated at 142A, into which the lower end of the main bore 130 opens, which may serve as an exhaust passageway for high pressure gases which result during the operation of the fuse unit 100 and which also serves as a chamber in which the fusible means 160 is disposed. The retaining member 142 also includes a relatively smaller opening 142B which extends axially therethrough into which the lower end of the auxiliary bore 192 opens and into which the lower end of the auxiliary conductor 182 projects. The insulating wall 142C around the relatively smaller opening through the retaining member 142 assists in preventing certain arc products which may result during the operation of the fuse unit 100 in the relatively smaller opening of the retaining member 142 from being deflected into the relatively larger opening of the retaining member 142 as indicated at 142A and impinging on parts of the fusible means 160. The retaining member 142 also includes an upwardly projecting portion 142D adjacent to the relatively smaller opening 142B through the retaining member 142 with the projecting portion 142D being joined to the adjacent block 128 around a recess in the block 128 which is adapted to receive the projecting portion 142 by a flexible bonding material, such as silicone rubber. This joint between the retaining member 142 and the block 128 around the auxiliary bore 192 serves to prevent the travel of ionized gases between the auxiliary bore 192 and the main bore 130 and between the auxiliary bore 192 and the relatively large opening through the retaining member 142 during an interrupting operation of the fuse unit 100i.

The elongated conducting member or rod S3 as shown in FIG. 2 is normally disposed to extend through the main bore 130 with the upper end of the conducting rod 83 projecting beyond the upper end of the tube 110 and with the upper portion of the conducting rod 83 being externally threaded, as indicated at 83A. The conducting rod 83 is normally held in the position shown in FIG. 2 by a connection through the fusible means 160 to the lower tubular conducting member or contact 150. More specically, the fusible means 160 comprises a strain element 162 and a fusible element `or link 164. The upper end of the strain element 162 is secured by suitable means, such as brazing, to the lower end of the conducting rod 83 while the other end of the strain element 162 is secured by suitable means, such as brazing, to the connecting conductor or terminal 156 which is of the flat strip type. The connecting conductor 156 is secured in turn to the lower conta-ct 150 adjacent to the upper end of the lower contact 150 by suitable means, suc-h as brazing. The upper end of the fusible element or link 164 is similarly secured to the lower end of the conducting rod 83 by suitable means, such as brazing, while the lower end of the fusible element or link is secured to the lower contact 150 adjacent to the upper end of the lower contact 150 by suitable means, such as brazing. It is to be noted the the strain element 162 and the fusible element 164 are electrically connected in parallel between the lower end of the conducting rod 83 and the tubular conducting member or contact 150.

Similarly, the auxiliary conductor 182 normally extends through the auxiliary bore 19.2 with the upper end of the auxiliary conductor 182 extending beyond the upper end of the auxiliary bore 192 and being both mechanically and electrically connected to the conducting rod 83 by a traversely extending spring pin 184 whic-h is disposed in a transversely extending recess provided at the upper end of the retaining member 189. The upper end of the auxiliary conductor 182 may be formed as a loop which is assembled over the spring pin 184 and retained thereon by the head 186 of the spring pin 184. The lower end of the auxiliary conductor 182 extends yor projects into the relatively smaller opening of the retaining member 142, as shown in FIG. 2, and is electrically connected through a helical conducting wire of reduced cross-section, as indicated at 194 to an angle-shaped auxiliary stationary terminal 157 which is secured to the tubular conducting member 150 adjacent to the upper end by suitable means, such as brazing as described and claimed in copending application Ser. No. 663,127, tiled Aug. 24, 1967, by C. W. Upton, Ir., C. C. Patterson and F. L. Cameron which is assigned to the same assignee as the present application The upper end of the helical wire 194 is secured to the lower end of the auxiliary conductor 182 by suitable means, such as brazing, and the lower end of the helical wire 194 is secured to the auxiliary terminal 157 by suitable means, such as crimping or brazing.

In order to assist in retaining the blocks 122 through 128, as well as the retaining member 142, in assembled relationship inside the tube during an interrupting operation of the fuse unit 100, the lower tubular conducting member or contact includes a main portion, as indicated at 152 which extends axially inwardly from the lower end of the tube 110 to bear against the lower end of the retaining member 142, as shown in FIG. 2. The lower conducting member or contact 150 also includes a flange portion 154 at the lower end thereof against which the lower end of the tube 110 bears when the conducting member 150 is assembled with the fuse tube 110. In order to retain the internal conducting member 150, as well as the other parts of the renewable unit 20, in assembled relationship, a generally tubular external terminal member or ferrule 172 is disposed to telescope over the lower end of the tube 110. In order to firmly secure the external terminal member or ferrule 172. to the lower end of the tube 110, the internal surface of the external terminal member 172 and the external surface of the portion of the adjacent tube 110 include adjacent helical grooves which when the parts are assembled form a passageway in which a helical wire 173 is disposed. In the assembly of the external terminal member 172 on the lower end of the tube 110, the helical wire 173 may be first assembled in the helical groove on the lower end of the tube 110 and the external terminal member 172 then screwed on to the lower end of the tube 110 until the parts reach their linal positions, as shown in FIG. 2. It is to be noted that the external terminal members 172 includes an inwardly projecting flange portion 172A around a central opening, as indicated at 172C, which bears against the adjacent llange portion 154 of the tubular conducting member 150 to assist in retaining the tubular connecting member 150 in assembled relationship with the other parts of the renewable unit 20. The external terminal member 172 also includes an external shoulder portion, as indicated at 172B, which bears against the inwardly projecting fiange portion 36B of the lower end tting 36 to form a conducting path which extends between the lower end fitting 36 and the external terminal member 172. A conducting path is also formed between the terminal member 172 and the internal tubular conducting member 150 through the inwardly projecting flange portion 172A of the external terminal member 172 which bears against the flange portion 154 of the tubular conducting member 150. The area of the current transfer path between the external terminal member 172 of the renewable unit 20 and the lower end fitting 36 is also augmented by the contact ring 175 which is formed of electrically conducting material and 7 which is disposed to threadedly engage the internally threaded opening at the lower end of the holder 32 and to bear against the terminal member 172 of the renewable unit 20.

As best shown in FIG'. 3, thev internal tubular conducting member 150 also includes an upwardly projecting arcing terminal 158 which projects upwardly from the upper end of the tubular conducting member 150 to overlap the lower end of the auxiliary conducting member 182 as best shown in FIG. 2 as described and Claimed in the last-mentioned copending application filed by C. W. Upton, Jr., et al. The arcing terminal-158 is normally electrically insulated by a coating of film of electrical insulation, such as an insulating enamel, or heat shrinkable tubing provided on the arcing terminal 158 to prevent the electrical shorting out of the helical Wire 194. The arcing terminal 158 which is formed from an electrically conducting material may be structurally secured to the tubular conducting member 150, as best shown in FIG. 3, adjacent to the upper end thereof at the inner periphery by suitable means, such as brazing.

It is important to note that in order to prevent the concentration of relatively high potential stresses adjacent to the external terminal member or ferrule 172 during an interruption by the fuse unit 100 at relatively high volt ages, the upper end of the tubular conducting member 150 extends axially beyond the upper end of the external terminal member 172 toward the other end of the tube 110 a minimum distance to prevent a concentration of relatively high potential stresses externally of the tube or casing 110 adjacent to the external terminal member 172. Such a concentration of relatively high potential stresses could cause a breakdown of the air or space adjacent to the outside of the tube or casing 110 adjacent to the terminal member 172 and possibly a fiashover which might extend the entire length of the tube 110 during an interrupting operation of the fuse unit 100. In addition, it is important to note that the internal tubular conducting member 150 terminates short of substantially the entire fusible means 160 in order to provide as compact a construction of the fuse unit 100 whose length might otherwise have to be increased in order to provide a sufficient insulating gas between the separated conducting parts of the fuse unit following an interrupting operation. In other words, the fusible means 160, as well as the helical wire 194 of the auxiliary conducting circuit previously described is substantially entirely exposed to electrically insulating material inside the renewable unit 20 prior to any interrupting operation, and that the disclosed construction avoids any axial overlapping of the internal conducting member or shield 150 and the fusible means 160 or the helical wire 194 which electrically connects the auxiliary conductor 182 to the auxiliary terminal 157, as previously described.

In order to actuate the axial movement of the conducting rod 83 during an interrupting operation of the fuse unit 100 and to electrically connect the renewable or refllable unit 20 just described to the upper end fitting orterminal 34 previously mentioned, a spring and cable assembly 30 is disposed inside the outer holder or tube 32 between the renewable unit 20 and the upper end iitting 34. The spring and cable assembly 30 includes at its lower end a generally tubular conducting member or vsocket 84 having an internally threaded central opening, as indicated at 84A, to receive the upper threaded end 83A of the conducting rod 83. A lower spring seat member 86 is fixedly mounted on the socket 84 for movement therewith by assembling the spring seat 86`over the outer periphery of the socket 84 with the lower end of the spring seat 86 resting on a shoulder provided on the outer periphery of the socket 84 and with the upper end being engaged by a plurality of portions at the upper end of the socket 84 which serve to stake the spring seat 86 on the socket 84. The springand cable assembly also includes an upper spring seat 74`whichA is slidably disposed over the lower portion 60A of a generally cylindrical conducting member 60 whose integral upper portion 60B extends axially through the opening 34B in the upper end fitting 34 and is externally threaded at the upper end thereof, as indicated at 60C. As illustrated, the generally cylindrical conducting member 60 is secured to the upper end fitting 34 by an internally threaded end cap 44 which may be screwed down on the threaded portion 60C of the conducting -member 60 until the flanged portion 44A of the end cap 44 bears against the upper end fitting 34 around a flange or shoulder portion, as indicated at 34C in FIG. 2. The helical tension spring 76 is secured at its upper end to the external helically threaded portion of the upper spring seat 74, while the lower end of the spring 76 is secured to the external helically threaded portion of the lower spring seat 86 to bias the conducting rod 83 as well as the auxiliary conducting member 182 in a generally upward direction, as viewed in FIG. 2, away from the tubular conducting'member of contact 150. It is important to note that the turns of the spring 76 are generally rectangular or square in crosshsection to substantially prevent any overlapping of the turns of the spring 76 during an interrupting operation of the fuse unit 100, as explained in greater detail in copending application Ser. No. 663,021 filed Aug. 24, 1967, by C. C. Patterson which is assigned to the same assignee as the present application.

In order to electrically connect the renewable unit 20 and more specifically the conducting rod 83 to the upper end fitting 34 both prior to an interrupting operation and during an interrupting operation, a plurality of helically coiled flexible cables or conductors 82 are electrically and structurally connected at the bottom ends thereof to the conducting socket 84 by suitable means, such as brazing or by staking, and at the upper ends thereof are secured to the conducting member 60 by suitable means, such as brazing or staking. In order to increase the effective current transfer area between the conducting member 60 and the upper end fitting 34, a washer 54 formed of electrically conducting material may be disposed between the shoulder which is formed at the intersection of the upper and lower portions 60A and 60B oty the conducting member 60 and the shoulder which is formed inside the upper end fitting 34 as indicated at 34D around the central opening 34B. In order to facilitate the assembly of the renewable unit 20 andthe associated spring and cable assembly 30 inside the outer holder 32, as will be explained hereinafter, a pair of spring pins S8 may be disposed in associated openings provided at the opposite sides of the upper portion 60B of the conducting member 60 to be positioned finally within an enlargedcentral opening or recess 34E in the upper end fitting, as shown in FIG. 2.

In "order to actuate the release of the latching assembly 360 shown in FIG. 1 following an'interrupting operation by the fuse'unit 100, a tripping rod or member 52 is slidably disposed inside a central opening or passageway 72 which is provided in the conducting member` 60 with the upper end of the tripping rod 52 being normally positioned below the top of theend cap, as shown in FIG. 2'. The lower end of the tripping rod 52 is fixedly coupled to the upper spring seat l74 for axial movement therewith by the cross pin 56 which passes laterally through aligned transverse openings in the tripping rod 52 andthe upper spring seat 74. The cross pin 56 also passes laterally through a pair of elongated slots 62 provided on Athe opposite sides of the conducting member 60 with the cross pin v56 normally positionedf'at the lower end ofieach of the elongated slots 62, asshown in FIG. 2. In o-rder to permit the axial movement of the tripping rod '52 upwardly through the Vend'cap 44, the top of theend cap 44 includes a central opening 46 through which thetripping rod 52 mayvpass upwardly to release the latching assembly 360 shown in FIG. 1. When the latch assembly'360 is releasedby theimovement of the tripping rod 52,"the upper end of the fuse unit will'be actuated to rotate in a clockwise direction, as viewed in FIG. 1, about the lower hinge assembly 350 to thereby provide an insulating gap between the upper contact 362 and the lower stationary contact 352 by such drop-out action.

In order to assemble the renewable or refillable unit and the associated spring and cable assembly within the associated outer holder 32, the threaded end of the conducting rod 83 of the renewable unit 20 is first screwed into the socket 84 at the lower end of the spring and cable assembly 30. A refill fusing tool (not shown) is then screwed into the internally threaded central opening or passageway 72 at the other end of the spring and cable assembly 30. The spring and cable assembly is then inserted into the outer holder 32 with the upper end of the spring and cable assembly 30 being inserted first into the lower end of the outer holder 32, as viewed in FIG. 2, until the refill tool (not shown) passes through the central opening 34B of the upper end fitting 34. By use of the refill tool, the spring 76 is stretched and placed in tension until the cross pins 58 are drawn upwardly through a pair of radial slots (not shown) provided in the upper end fitting 34 around the central opening 34B. The upper conducting member and the spring and cable assembly 30 are then rotated until the pins 58 rest on the shoulder provided at the bottom of the enlarged opening 34E in the upper end fitting 34. The end cap 44 may then be screwed down on the upper threaded portion 60C of the conducting member 60 to further stretch the spring 76 to the final position shown in FIG. 2, in which the cross pins 58 are drawn upwardly away from the shoulder in the upper end fitting 34 at the bottom of the enlarged opening 34E. It is to be noted that when the spring and cable assembly and the associated renewable unit 20 are assembled inside the outer holder 32 as just described, the cross pin 56 which couples the upper spring seat 74 to the tripping rod 52 is disposed at the bottom of each of the elongated axially extending slots 62 at the opposite side of the conducting member 60 to permit limited upward travel of the upper spring seat 74 along with the cross pin 56 and the tripping rod 52 to a final position of the tripping rod 52 in which the tripping rod 52 projects beyond the end cap 44 to release the latching assembly 360, as previously mentioned with the upper end of each of the slots 62 acting as a stop means to limit the upward travel of the cross pin 56 and the upper spring seat 74.

In considering the operation of the fuse unit 100, it is to be noted first that the current paths which include respectively the strain element 162, the fusible element 164 and the helical wire 194, which is connected in series with the auxiliary conductor 182, are all electrically connected in parallel between the upper end of the conducting rod 83 and the internal tubular conducting member or shielding member 150 at the lower end of the renewable unit 20. The resistance of the current path which includes the fusible element 164 is arranged to be relatively much less than the resistance of either the path which includes the strain element 162 or the path which includes the helical wire 194 so that normally most of the current which flows through the fuse unit 100 is carried by the fusible element 164. Although the resistance of the current path which includes the strain element 162 is relatively greater than the resistance of the path which includes the fusible member 164, the resistance of the path which includes the strain element 162 is relatively much less than the resistance of the path which includes the helical wire 194 so that when the fusible element 164 melts or blows, most of the current which was formerly carried by the fusible element 164 will be transferred to the strain element 162. In other words, when the current which is fiowing through the fuse unit 100 increases to a value which is of sufficient magnitude and duration to melt or blow the fusible element 164, most of the current which is flowing through the fuse unit 100 then transfers to or is carried by the strain element 162. When the current which is transferred to the strain element 162 is suicient to melt or 10 blow the strain element 162, the current which was previously carried by the strain element 162 is finally transferred to the current path which includes the auxiliary conductor 182 and the helical wire 194. It is to be noted that when the strain element 162 melts or blows, the conducting rod 83 is no longer restrained from upward movement under the inuence of the biasing spring 76 and the conducting rod 83 will start to move upwardly under the influence of the biasing spring 76 to thereby stretch the helical wire 194 which is electrically connected to the bottom of the auxiliary conductor 182. It is also to be notedv that while the current path which includes the auxiliary conductor 182 and the helical wire 194 is intact that no arcing will take place either in the main bore 130 or in the auxiliary bore 192.

After the strain element 162 melts or blows, and the arcing rod 83 begins to move upwardly to thereby stretch the helical spring 94, the helical Wire 194 will either fracture mechanically when stretched to its limit or the current transferred to the current path which includes the auxiliary conductor 182 and the helical wire 194 will be sufficient to melt or blow the helical wire 194 which is of reduced cross-section compared with the main auxiliary conductor 182. After the helical wire 194 is melted or otherwise broken, an arc will be initiated in the auxiliary bore between the retreating end of the broken helical wire 194 and the arcing terminal 158 which axially overlaps the lower end of the auxiliary conductor 182. The electrical insulation on the arcing terminal 158 will then burn through. The formation of a significant insulating gap in the auxiliary current path is delayed by the overlapping of the auxiliary conductor 182 by the arcing terminal 158 until the retreating free end of the broken or melted helical wire 194 passes the upper end of the arcing terminal 158 whose insulation will have burned through by this time. It is important to note that the insulating gap in the main bore 130 between the melted end of the strain element 162 which is secured to the lower end of the conducting rod 83 will increase at a faster rate than the formation of an insulating gap in the auxiliary current path just described due both to the delay in the formation of an arc in the auxiliary current path because of the presence of the helical wire 194, as just described, and due to the overlapping of the arcing terminal 158 at the lower end of the auxiliary conductor 182. It is important to note that when any abnormal current flows in the fuse unit which is sufficient to melt or blow the fusible element 164 and the strain element 162 and the melting or breaking of the helical wire 194 results in the auxiliary current path, any arcing which takes place in the fuse unit 100 will take place initially in the auxiliary bore 192 as just explained. When the retreating end of the broken helical wire 194 passes the upper end of the arcing terminal 158, the arcing which takes place initially in the auxiliary bore 192 will cause gases to be evolved from the gas evolving material around the auxiliary bore 192 which will be unionized. As the auxiliary conductor 182 moves upwardly inside the auxiliary bore 192 and the gas pressure of the evolved gases increases to thereby increase the dielectric strength in the auxiliary bore 192, for certain abnormal or fault currents of a relatively lower magnitude, the insulating gap which is formed in the auxiliary bore 192, along with the increased dielectric strength, will be sufficient to interrupt the alternating current following a particular current zero during the formation of an insulating gap in the auxiliary bore 192.

For relatively higher current faults, the gas pressure which builds up in the auxiliary bore during an interrupting operation, as just described, will be sufficiently high to result in a relatively higher dielectric strength in the auxiliary bore compared with the dielectric strength in the main bore between the upwardly moving conducting rod 83 and the stationary broken ends of the strain element 162 and the vfusible element 164. If the mstantaneous potential difference between the separating conducting parts in the main bore 130, such as the broken end of the moving portion of the strain element 162 and the relatively stationary portion remaining attached to the connecting conductor 156 is suiiicient, the arc will restrike in the main bore 130 to thereby cause the evolution of unionized gases to increase the gas pressure in the main bore 130 and the associated dielectric strength to finally interrupt the current in the main bore 130 following a particular current zero in the alternating current which is being interrupted. If the fault current which is being interrupted is of a relatively still higher magnitude, the gas pressure in the main bore 130 may be suicient to disintegrate the inner walls of the blocks 126 and 128 to thereby limit the gas pressure of the evolved gases to a value within the rupture strength of the tube 110 as previously explained. When the arc is interrupted in the main bore 130 to thereby cause the evolution of gas from the gas evolving material in the blocks 122, 124, 126 and 128 which surround the main bore 130, the upward movement of the conducting rod 83, along with the upward movement of the auxiliary conductor 182, will be accelerated by the gas pressure of the evolved gases in the main bore 130 along with the force exerted on the conducting rod by the biasing spring 76.

During an interrupting operation of the fuse unit 100 as just described, when the conducting rod 83 is released and moves upwardly under the influence of the biasing spring 76 or under the influence of both the biasing spring 76 and the gas pressure of the evolved gases inside the yrenewable unit 20, the turns of the spring 76 which are normally held in tension will tend to collapse toward a compressed condition, but after the turns of the spring 76 partially collapse, the upper spring seat 74 will begin to slide axially on the conducting member 60. The upper spring 74 will continue to slide axially on the conducting member 60 until the upper end of the spring seat 74 impacts against the washer 54 to thereby drive the tripping -rod 52 in an upward direction from the position shown 1in FIG. 2 until the upper end of the tripping rod 52 actuates the release of the latching means 360, `as `described in the copending application previously mentioned `and in U.S. Patent 2,403,121 which issued July 2, 1946 to H. L. Rowlins et al. The upward movement of the spring seat 74 is limited, as previously mentioned, by the washer 54 which acts as a stop against which the upper end of the spring seat 74 impacts during the iinal collapse of the turns of the spring 76 to a fully compressed condition to thereby actuate the conducting rod 83 and the auxiliary conductor 182 upwardly to establish insulating gaps betweenthe separated ends of the conducting parts inside the renewable unit following an interrupting operation. As previously mentioned, the fuse unit 100 will be actuated by the release of the latching mechanism 360 to rotate in a clockwise direction about the hinge assembly 350 in a drop-out movement.

During an interrupting operation as just described, if the upper end of the internal tubular conducting member 150 did not extend axially beyond the upper end of the external terminal member 172 to provide shielding for the external terminal member 172 around the inner periphery of the fuse tube 110 adjacent to the upper end of the external terminal member 172, an external flashover could result adjacent to the external terminal member 172 in the air or space adjacent to the external terminal member 172, due to the instantaneous concentration of potential stresses between the external terminal member 172 and the retreating conducting parts of either the main conducting rod 83 or the auxiliary conductor 182. Since the arcing which is initiated during an interrupting operation ofthe fuse unit 100 may persist through one or more half cycles of the alternating current which is being interrupted until dielectric conditions result within the fuse unit 100 which `are adequate to interrupt the arcing, following a particular current zero, the system voltage at the instant of interruption is very near its peak value and the fuse unit 100 is subjected to relatively high, oscillatory voltage peaks which could result in a tiashover 4adjacent to the external terminal member 172 in the absence of the inter-nal tubular conducting member 150, considering the instantaneous position of the separating conducting parts during an interrupting operation as described. If`the instantaneous electric field strength adjacent tothe external terminal member 172 is suicient following an interruption of the arc considering the -relative positions of the separated conducting parts, a iiashover could beinitiated in the space adjacent to the external terminal member 172 which could lengthen to include the entire external surface of the tube 110 to establish a ashover which the fuse unit is not capable of interrupting since the arc extinguishing means of the fuse unit 100 is located internally of the tube 110. By providing the internal tubular conducting member or shield 150, any electric field present during an interrupting operation of the fuse unit 100, will terminate either on the tubular conducting member 150 or on one of the projecting conducting parts, such as the arcing terminal 158 or the conducting member 156. The concentration of potential stress during an interrupting operation will thus be transferred inside the tube where the dielectric strength is suicient to prevent any restrike of the are following an interrupting operation. It is important to note that the tubular conducting member should extend axially beyond the upper end of the external terminal member 172 only a minimum `distance which is just sufficient to prevent the external liashoversof the type described, since if the tubular conducting member 150 were extended farther to axially overlay a substantial portion of the fusible means 160 or the helical wire 194, the length of the fuse unit 100 would have to be increased in order to insure an adequate insulating gap between the separated conducting parts of the fuse unit following an interrupting operation to support the potential difference which results between the separated conducting parts following such an interrupting operation. In the applicants disclosed arrangement, the location of the internal parts of the fuse unit 100 insures that substantially the entire fusible means 160 is exposed to electrically insulating surfaces at the inner periphery of the tube 110 in the normal operating condition of the fuse unit 100', as shown in FIG. 2. It is also important to note that the ends of the moving main and auxiliary conductors are spaced farther away from the external end cap in the applicants disclosed arrangement at the time that the lfirst current zero occurs following separation of the conducting parts during an interrupting operation. Increasing the distance between the ends of the main and auxiliary conductors and the external end terminal 172 prior to the occurrence of the first current zero also decreases the possibility of a flashover adjacent to the external terminal member 172 during an interrupting operation of the fuse unit 100.

It is to be understood that the teachings of the applicants invention may be applied to power fuses for high voltage applications which are of the non-dropout type as disclosed in copending application Ser. No. 663,021, iiled Aug. 24, 1967, by C. C. Patterson which is assigned to the same assignee as the present application. It is also to be understood that a displaceable end cap (not shown) maybe lremovably secured to the lower end of the outer holder 32 to normally close off the opening in the lower end of the holder 32.

r[lhe apparat-us embodying the teachings of this invention has several advantages. For example, ashovers external tothe insulating casing due to the concentration of relatively high potential stresses d-uring an interrupting operation of a power fuse intended for high voltage application is prevented while simultaneously minimizing the overall size of the Vfuse unit that would otherwise be required. In addition, the internal tubular conducting member which is employed for shielding purposes also provides an exhaust passageway of increased size for the gas pressures which result during an interrupting operation of the disclosed fuse unit. yIt is to be noted that the size or cross-section of the internal opening provided by the tubular conducting member 150, as disclosed is substantially uniform from the lower end of the associated insulating casing 110 to substantially the upper end of the conducting member 150 adjacent to which the fusible means of the associated main and conducting paths are connected. Finally, the internal tubular conducting member 150 'as disclosed assists in maintaining the internal parts of the fuse unit of which it forms a part in assembled relationship during an interrupting operation of the fuse unit.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A high voltage circuit interrupter comprising a tubular insulating casing, terminal means disposed adjacent to .at least one end of said casing, said terminal means being generally tubular and telescoping over one end of said casing to exten-d axially a predetermined distance along the casing toward the other end of said casing, a body of arc extinguishing material disposed inside of and spaced from the ends of said casing around a central passageway, a conducting member disposed to move axially through said passageway land extending axially through said central passageway toward said terminal means, fusible means disposed inside the casing and connected at one end to said movable conducting member and extending axially away from said movable conducting member, and a tubular conducting member electrically connected adjacent to one end thereof to said terminal means and adjacent to the other end thereof to the other end of said fusible means, said tubular conducting member extending inside `said casing adjacent said terminal means axially 'beyond said one of the terminal means toward the other end of said casing a minimum distance which is just suicient to prevent a ashover externally` of the casing adjacent to said terminal means when the fusible means blows.

v2. The combination as claimed in claim 1 wherein substantially the entire fusible means is exposed to axially extending, electrically insulating material inside the caslng.

3. The combination as claimed in claim wherein a lining of electrically insulating material is disposed inside the casing and extends axially between the body of arc extinguishing material and the tubular conducting member around the fusible means with substantially the entire fusible `means being exposed to said lining of electrically insulating material.

4. The combination as claimed in claim 1 wherein the tubular conducting member which extends inside the casing terminates short of substantially the entire fusible means.

5. The combination as claimed in claim 1 wherein the size of the opening provided by the tubular conducting member to permit gas discharge during the operation of the circuit interrupter is substantially uniform 'from the end of the conducting member adjacent to the fusible means to the end of the conducting member which is electrically connected to said one terminal means.

6. A high voltage circuit interrupter comprising a tubular insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, one of the terminal means being generally tubular and extending axially over one end of the casing a predetermined distance along the casing toward the other terminal means, means for extinguishing an arc disposed inside of and intermediate the ends of said casing, said arc extinguishing means including an axially extending central passageway, an elongated movable conducting member electrically connected to the other terminal means and extending axially through the central passageway toward said one of the terminal means, means connected to one end of the conducting member for moving the conducting member away from said one of the terminal means, fusible means disposed inside the casing and connected at one end to the other end of the conducting member and a tubular conducting member electrically connected adjacent to one end thereof to said one of the terminal means and adjacent to the other end thereof to the other end of said fusible means, said tubular conducting member extending inside said casing axially beyond said one of the terminal means a minimum distance toward the other terminal means which is just suicient to prevent a ashover externally of the casing adjacent to said one of the terminal means when the fusible means fuses.

7. The combination as claimed in claim 6 wherein substantially the entire fusible means is exposed to axially extending, electrically insulating material inside the casing.

8. The combination as claimed in claim 6 wherein a lining of electrically insulating material is disposed inside the casing and extends axially between the arc extinguishing means and the tubular conducting member around the fusible means with substantially the entire fusible means being exposed to said lining of insulating material.

9. The combination as claimed in claim 6 wherein the fusible means includes a strain element to restrain the movement of the conducting member away from said one of the terminal means.

10. The combination yas claimed in claim 6 wherein the tubular conducting member which extends inside the casing terminates short of substantially the entire fusible means.

11. The combination as claimed in claim 6 wherein substantially the entire fusible means is disposed between the .are extinguishing means and the tubular conducting member.

References Cited UNITED STATES PATENTS 2,183,728 12/1939 Triplett 200-120 XR 2,502,992 4/1950 Rawlins et al. 200--120 XR 2,961,514 11/ 1960 Lindell 20G-l2() 2,976,381 3/1961 `Lindell 200-120 3,230,331 1/1966 Lindell 20G-120 2,599,186 6/ 1952 Lindell 200--120 XR BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner. 

